1. Field of the Invention
The present invention relates to a testing head having cantilever probes, and more particularly to a testing head for use on semiconductor integrated devices.
2. Description of the Related Art
As is well known, a testing head is basically a device suitable to electrically interconnect a plurality of contact pads of a microstructure and the corresponding channels of a testing machine that is to perform the tests.
Integrated circuits are factory tested in order to spot and reject any circuits which are already defective during the production phase. The testing heads are normally employed to electrically test the integrated circuits xe2x80x9con waferxe2x80x9d, prior to cutting and mounting them in a chip package.
As schematically shown in FIGS. 1 and 2, a testing head 1 having cantilever probes usually comprises a backing ring 2, made of aluminum or ceramics, to which a resin holder 3 is attached, and that is suitable to hold a plurality of movable contact elements or probes 4, being normally wires made of special alloys having good electrical and mechanical properties, the probes being mounted to jut out of the resin holder 3 at plural points 5 and at a suitable angle from a plane xcex2. Such emerging probes are commonly known as xe2x80x9ccantilever probesxe2x80x9d.
In particular, each probe 4 has an end portion or contact tip 6, which is bent with an angle xcex3 from the rest of the probe so that a plurality of contact pads 7 of a device to be tested is contacted. The bent contact tips 6 are commonly referred to as the xe2x80x9ccrooksxe2x80x9d.
The good connection of the probes 4 of the testing head 1 to the contact pads 7 of a device to be tested is ensured by the testing head 1 exerting a pressure on the device, whereby the probes 4 are vertically flexed in the opposite direction from the device movement towards the testing head 1.
As schematically shown in FIG. 3 for a single probe 4, as the device to be tested vertically moves against the contact tip 6, the probe 4 flexes, and its elbow point X, situated at the transition from the contact tip 6 to a probe section 8 emerging from the resin holder 3, describes a circular arc.
Thus, the jutting probe section 8 forms a working arm of the probe 4 adapted to flex vertically, and is commonly referred to as the xe2x80x9cfree lengthxe2x80x9d of the probe.
The crooked shape of the probes 4 is designed to allow the contact tips 6 of the probes 4 to skid, upon coming in touch with the contact pads 7 of the device to be tested and during the pad overtravel beyond a pre-set point of contact, across the contact pads 7 along a direction dictated by the arrangement geometry.
It should be noted that the force exerted on the contact pads 7 by each probe 4 depends on many factors, among which are especially the type of material forming the probe 4, the probe shape, the angle xcex1 made by the probe contact tip 6, the length of the probe jutting section or free length 8, and the amount of overtravel of the pads to be measured. These factors also determine the extent of the contact tips 6 skidding on the contact pads 7, this being commonly known as the xe2x80x9cscrubxe2x80x9d.
It should be noted that, with a dense distribution of the contact pads 7, the probes 4 must be arranged in plural rows, and the lengths L1, . . . Ln of the crooked ends vary accordingly, as schematically shown in FIG. 4.
Also known is to use backing rings 2, generally made of aluminum or ceramics, having different shapes depending on the set of contact pads 7 to be tested, so that the free lengths of the probes 4, and hence the forces exerted by the latter to the contact pads 7, can be equalized in the interest of even wear and performance of the testing head 1.
In particular, when the probes 4 are arranged in a plurality of rows or levels, as schematically shown in FIGS. 5A, 5B and 5C, the emerging points 5 of the probes 4 from the resin 3, when viewed frontally, make either a diagonal (FIG. 5A), or straight (FIG. 5B), or combination pattern (FIG. 5C) that is dependent on constructional requirements.
The portions of the probes outside the backing ring 2 are usually soldered on a PC board 9, as shown in FIG. 1, to establish an electrical connection between the testing head 1 having cantilever probes and the testing machine.
It is therefore necessary that the outer portion of any probe 4 can be recognized unfailingly in the probe bunch, so that it can be soldered on the PC board 9 in the correct manner.
In addition, the probes 4 extend with their sections outside the backing ring 2 parallel to one another, as shown in FIG. 6A (side A), and the probes 4 for soldering on the PC board 9 are not easily singled out. It is also known the use of probes 4 with a radial spreading in their portion outside the backing ring 2, as schematically shown in FIG. 6A (side B).
The probes 4 can be arranged in a plurality of rows or layers such that they have a diagonal or a straight configuration, in either the case of parallel or radial probes, as shown in FIG. 6A.
FIG. 6B shows, by way of example, an arrangement of the probes 4 in three rows with a radial diagonal configuration, and FIG. 6C shows an arrangement of the probes 4 in three rows with a radial straight configuration.
It is, moreover, a known fact that certain electronic devices, e.g. memories, have contact pads disposed along two sides only. Accordingly, a number of such devices can be tested in parallel if they are set in a single row.
A row of devices can be tested by suitably calibrating the inside dimensions of the backing ring 2.
When several rows of devices are to be tested in parallelxe2x80x94usually two rows of eight devices or four rows of eight devicesxe2x80x94multi-bridge backing rings, schematically shown in FIGS. 7A and 7B, are used.
In particular, a multi-bridge backing ring 2b includes a plurality of bridges 2c having a width dimension P inside the ring 2b perimeter, which bridges are suitable to carry probes for several devices to be tested in parallel. There are various techniques that can be used in order to obtain the desired pressure uniformity on the probes 4 against the corresponding contact pads 7.
A first known technique uses a multi-bridge backing ring 2b having plural bridges 2c inside its perimeter to define plural device rows FILA1, FILA2, . . . as schematically shown in FIGS. 7A and 7B.
The shape and dimensions of the multi-bridge backing ring 2b and the inner bridges 2c are selected such that the jutting sections or free lengths FL1, FL2, FL3, FL4, . . . of all the probes will be equalized. In this way, the probes 4 are all caused to abut on the contact pads 7 with the same force.
A limitation comes to this prior technique from that the minimum width Lmin of a device to be tested cannot be less than the sum of the minimum length FLmin of the jutting section or minimum free length FL1, FL2, . . . of the probes 4 and the minimum theoretical width Pmin of each inner bridge 2c, i.e.:
Lminxe2x89xa7FLmin+Pmin,xe2x80x83xe2x80x83(1)
as schematically shown in FIG. 7A.
A second prior technique uses probes of different types bound to the same backing ring 2, as schematically shown in FIGS. 8A and 8B.
In particular, probes 4 of a larger diameter are used for the innermost contact pads 7 within the backing ring perimeter, to have equal forces exerted on the contact pads 7 even though the jutting sections or free lengths FL1, FL2, . . . may be different.
Using this technique, however, the dimensions and free lengths of the probes 4 are difficult to calibrate for an even pressure on all the contact pads 7. In addition, the probes 4 which are to reach devices located farther inwards than the probe emergence points on the backing ring 2 will be those having the largest dimensions, as having the greatest jutting sections or free lengths 8, thus enforcing reduced density for the contact pads 7 on the devices.
Also, neither of the above prior techniques would work where a large number of devices are to be tested in parallel. In particular, the testing heads so provided cannot test more than two rows of devices, and are definitely incapable of testing a matrix array of devices.
Embodiments of this invention provide testing heads for microstructures, having a configuration which can facilitate the operations of sorting out probes and soldering them on a PC board, specifically densely clustered probes, and allowing an unlimited number of devices laid into plural rows to be tested.
One of the principles on which embodiments of the present invention stand is one of suitably shaping the resin holder attached to the probe backing ring in order to provide a clearer view of each probe in a cluster of probes during the soldering steps, while also maintaining an accurate control of the probe jutting sections or free lengths when a plurality of devices are under parallel test.
Presented is a testing head having cantilever probes and comprising a backing ring and a resin holder attached to the backing ring, as well as a plurality of contact probes held by the resin holder and formed with respective contact tips arranged to mechanically and electrically contact a plurality of contact pads of at least one device to be tested, the holder being formed with at least one suitably shaped outline to allow different probe rows to emerge in a cantilever manner. Additionally presented is a method of creating an electro/mechanical connection between a testing head and a test device.
The features and advantages of a testing head according to the invention will become apparent from the following description of embodiments thereof, given by way of non-limiting examples with reference to the accompanying drawings.